Peanut allergy (PA) Is an increasingly common form of food allergy associated with severe clinical reactions to peanut, Including fatal anaphylaxis. Recent studies in children (less than 18 years) have provided evidence that oral immunotherapy (OIT) can reduce the sensitivity of PA patients to peanut. However, it is not yet clear to what extent such patients have developed desensitization/non-tolerance, defined as a lack of clinical reactivity to peanut that requires ongoing exposure to peanut for this muted clinical reactivity to peanut to be maintained, as oppose to tolerance, in which the patient exhibits a long term and perhaps permanent loss of reactivity to peanut^^. In the Phase 2 clinical trial of peanut OIT that will be conducted in Project 1 of this U19 proposal, we will determine which subjects develop desensitization/non-tolerance vs. tolerance to peanut. In this project, we will investigate changes in the T cell populations of PA subjects who respond to OIT, in order to identify changes that might contribute to the development of tolerance. We particularly will focus on changes in regulatory T cells (Treg) in PA patients who exhibit favorable responses to OIT. After characterizing and quantifying the modulation of T cell phenotype and function associated with peanut OIT, we will then test the results of these experiments for possible correlation with the development of tolerance vs. non-tolerance in PA patients who respond favorably to peanut OIT. Our main hypothesis is that memory T effector cells (Teff) specific to peanut allergen are converted Into Treg through epigenetic changes in the FOXPS locus that increase FOXPS expression, resulting In improved Treg function, as reflected In a Treg-dependent reduction of Teff proliferation in response to peanut. To test this hypothesis, we propose to: (Aim 1) Characterize the immunophenotypic and functional changes induced by OIT in specific T cell populations in patients with PA; (Aim 2) Quantify epigenetic changes (I.e., methylation of CpG islands) in key loci (i.e., F0XP3) to assess whether such changes are associated with enhancement of Treg function and favorable clinical outcomes in OIT; and (Aim 3) Use MHC class II multimer-based methods to characterize the specificity of T cell recognition of peanut peptides. If we achieve these aims, we expect our results to provide new insights into the mechanisms underlying tolerance to food allergens, and to Improve understanding ofthe immune changes that contribute to the durability and safety of oral Immunotherapy.